Polyester-polyurethane hybrid resins are known in the art of thermoset molding compositions (e.g., U.S. Pat. No. 5,153,261). These resins are normally tougher than polyesters and stronger, stiffer and less expensive than polyurethanes. Typical of such resins are those comprising a hydroxy-terminated unsaturated polyester polyol, an ethylenically unsaturated monomer (e.g., styrene) and a polyisocyanate. They are easily adapted to many common thermoset molding techniques presently employed in the polyurethane and unsaturated polyester industries. Xycon® hybrid resins available from Cook Composites and Polymers are representative of these resins.
Hybrid resins are two component or part systems comprising an A part and a B part. The A part contains the polyisocyanate and a polyester catalyst, while the B part contains the hydroxy-terminated unsaturated polyester polyol/unsaturated monomer solution, optionally with a polyurethane catalyst and/or filler. Upon mixing parts A and B together under the appropriate conditions, an interpenetrating network of molecular chains is formed. The polyester component of the mix provides the chain extension function while the polyisocyanate component provides the crosslinking function. The result is a molded part or coating that demonstrates improved toughness and thermal properties over either component alone.
Gel coats are typically used as the outer or external surface layer of composite molded article because they impart a smooth, durable appearance to the article. Unsaturated polyesters resins are widely used for marine and cultured marble gel coats because they are inexpensive, easy with which to work, and cure at room temperature. Moreover, these resins provide a strong, flexible, abrasion and impact resistant surface. However, these coating properties require improvement in certain stressful applications, such as windmill blades. These applications require a coating with superior moisture-resistance, toughness (e.g., resistance to cracking) and similar properties that will protect the underlying laminate from deterioration by environmental forces.